The present invention relates to an optical amplifier applied to an optical transmission apparatus such as an optical transmitter/receiver apparatus or an optical repeater in an optical communication system.
In the prior art, a dispersion compensating fiber is used in an optical receiver or an optical transmitter in order to suppress a waveform distortion of an optical signal due to a waveform dispersion of a transmission line fiber, but because of its large loss, it is essential to use it with an optical amplifier for compensating the loss. This technique is disclosed in OSA Optical Fiber Communication Conference, 1992, pp. 367-370.
FIG. 14 shows a configuration of an optical fiber transmission system which uses a prior art dispersion compensating optical transmitter 100 and a prior art dispersion compensating optical receiver 200. The optical transmitter 100 comprises an erbium doped optical fiber amplifier 101a, a dispersion compensating fiber 103a and an electro-optical converter 104. The optical receiver 200 comprises erbium doped optical fiber amplifiers 201a and 201b, optical band-pass filters 202a and 202b, dispersion compensating fibers 203a and 203b and a photo-electric converter 205. Losses of the dispersion compensating fibers used are 3.1 dB, 10.6 dB and 5.3 dB, respectively for a light signal level. In order to compensate for the losses, a total of three erbium doped optical fibers are used, which amplify the signal lights by using separate pumping light sources. A characteristic of the optical fiber amplifier when the dispersion compensating fiber is added is that a noise figure is increased by a loss when the dispersion compensating fiber is arranged in a preceding stage, and a light output is decreased by the loss when the dispersion compensating fiber is arranged in a succeeding stage.
In the known dispersion compensating optical transmitter and dispersion compensating optical receiver shown in FIG. 14, the light signal level is lowered because of a large loss of the dispersion compensating fiber. When the optical amplifier is used to compensate for the loss, the light output of the optical amplifier decreases and the problem of increase of the noise figure arises. In order to avoid the problem, it is necessary to arrange a separate optical amplifier.